1. Field of the Invention
The invention relates to thermistors and somewhat more particularly to thermistors composed of a sintered oxide mixture containing rare earth metals.
2. Prior Art
Resistor elements which possess a negative temperature coefficient of resistance are generally referred to as thermistors or NTC-resistors. Generally, such thermistors are composed of sintered oxides of manganese, iron, cobalt, nickel, copper, zinc and mixtures thereof.
Thermistors composed of such metal oxides and mixtures thereof are typically not suited for high temperature applications because these metal oxides and mixtures thereof decompose at temperatures above about 600.degree. C. Further, since irreversible changes may occur within thermistors of this type even at lower temperatures, the prior art has generally limited their usage to maximum temperatures in the range of about 300.degree. to 350.degree. C.
Measurement and/or control of temperatures in the range of about 600.degree. to 1100.degree. C. was heretofore effected by utilizing either pyrometers, metal resistors or thermocouple elements. However, pyrometers are characteristically relatively inaccurate for temperature measurement so that it is virtually impossible to use pyrometers for accurate temperature control. Metal resistors have a low temperature coefficient of resistance so that amplifiers are typically required whenever metal resistors are utilized for temperature control. Finally, thermocouple elements suitable for high temperature environments can only be produced from relatively high-costing platinum metals.
The prior art is aware that thermistors usable at higher or high temperatures may be produced from a mixture of a rare earth metal oxide and zirconium oxide. For example, British patent specification No. 874,882 suggests a thermistors comprised of a mixture of yttrium and zirconium oxide while German Offenlegungsschrift No. 2,333,189 suggests a thermistors composed of a mixture of praseodymium and zirconium oxide. However, thermistors composed of such materials exhibit a varistor effect, i.e., the resistance values of such thermistors is dependent not only upon the temperature but also on the applied voltage.
"Zeitschrift fur Electrochemie" (Journal for ElectroChemistry) 1959, pages 269-274, suggests that the conductivity of rare earths increases with rising temperatures, however, no suggestions are made for incorporating rate earths or mixtures thereof in thermistors.
Further, U.S. Pat. No. 4,010,119 suggests a thermistor comprised of a mixture of neodymium oxide and samarium oxide while U.S. Pat. No. 4,010,122 suggests a thermistor composed of a mixture of terbium oxide and erbium oxide. However, thermistors composed of such materials exhibit a relatively high specific resistance and thus cannot be utilized at relatively low temperatures or over an extended operating temperature range.
Thus, previously known thermistors useful in high temperature environments are either too highly ohmic, have too low of temperature coefficients of the resistance value, exhibit varistor effects and polarization phenomena or have too narrow of an operative temperature range.